Heat 2cm of oil in a large pot or frying pan. Fry the aubergine cubes in batches, until golden brown. Drain on paper towels.
Heat another saucepan over a medium heat with a little oil and sauté the onions for 30 minutes or until soft and caramelised.
Preheat the oven to 180°C, gas mark 4. Line a large 26cm x 20cm baking tray with baking paper.
Whisk the eggs, yoghurt, flour and baking powder together in a bowl. Stir in the cooled onion, aubergine, feta and basil. Season well.
Pour the mixture into the prepared baking tray and bake for 30 minutes until golden brown.
Allow to cool slightly then cut into triangles.
Serve hot or cold garnished with extra feta and basil.